1. Field of the Invention
This invention relates to a position sensor for detecting a magnetic element.
2. Discussion of the Related Art
For detecting the position of an object, some transducers exploit the Hall effect, whereby, if a conductor material through which current flows in one direction is immersed in a magnetic field directed perpendicular to the current flow direction, across the sensor an electric field is observed perpendicular to both the current and the magnetic field and proportional to the strength of the magnetic field at that point.
Consequently, with known Hall-effect transducers, by measuring the potential difference across the sensor, it is possible to determine the position of a magnetic element (e.g., a permanent magnet or magnetic circuit) whose field pattern is known.
Generally, as the magnetic field of the magnetic element varies in space, typical sensors only provide for accurate determination of position when the magnetic element is moved solely in a given straight line, typically parallel or perpendicular to the sensor. Any unpredictable component in the movement of the magnetic element introduces measuring errors, thus impairing the reliability and limiting the scope of known Hall-effect transducers.
Further limitations are posed by the sensitivity of Hall-effect transducers to any misalignment of the magnetic element and sensor, and to variations in the quantities affecting the transfer constant, such as supply voltage and current, and the characteristics of the sensor (e.g. in the case of solid-state sensors, carrier mobility being dependent on temperature). As a result, known transducers involve mechanical alignment, electrical calibration, and highly complex circuitry for compensating for variations in operating conditions such as temperature. Even these provisions do not always succeed in achieving the desired accuracy and reliability in the transducer.
To solve these problems, an array of Hall type elementary sensors has been employed to determine the zero point of the field component and to generate directly at the output a numeric code indicating the zero point position (see, e.g. EP-A-0 427 882).
Moreover, it is known to integrate a single Hall effect cell in an integrated circuit (see e.g. G. Bosch, "A Hall device in an integrated circuit", Solid State Electron, vol. 11, pages 712 to 714, 1968; G. S. Randhawa "Monolithic integrated Hall devices in silicon circuits", Microelectron. Journal, vol 12, pages 24 to 29, 1981).
Accordingly, commercial products, having the structure shown in FIG. 5, have been developed. These products include a substrate 50 (preferably of P-type), an epitaxial N-type layer 51 having depth t, a P.sup.+ -type isolation region 52 formed in the epitaxial layer 51 for delimiting an active region 53, and N.sup.+ -type contacts 54, 55 formed during the emitter diffusion step of the integrated circuit. The Hall voltage V.sub.H of such cell is: ##EQU1## wherein G is a geometrical correction factor due to the finite dimensions of the cell (G=1, by conveniently dimensioning the cell), N is the concentration of doping species of the epitaxial layer, t is the depth of the epitaxial layer, and B is the component of the magnetic induction which is perpendicular to the current I.